The present invention relates to a light-sensitive material for making lithographic printing plates which is suitable to be exposed with a laser beam as a light source and in particular to a lithographic printing plate and a method for making the printing plate by silver complex diffusion transfer process.
A lithographic printing plate which utilizes directly as ink receptive areas a transferred silver image produced by silver complex diffusion transfer process has been known by the disclosures of U.S. Pat. Nos. 3,728,114 and 4,134,769.
In a typical embodiment of the silver complex diffusion transfer process suitable for making lithographic printing plate, a silver image is formed in the following manner. When a light-sensitive material comprising a support and, provided thereon, a subbing layer which serves also as antihalation layer, a silver halide emulsion layer and a physical development nuclei layer is imagewise exposed and developed, the silver halide in the areas where a latent image has been formed changes into blackened silver in the emulsion layer. At the same time, the silver halide in the areas where a latent image has not been formed dissolves by the action of a silver halide complexing agent contained in the developer and diffuses towards the surface. The dissolved and diffused silver complex is precipitated by the reducing action of the developing agent on the physical development nuclei in the surface layer to form a silver image. After the development, if necessary, the silver image is subjected to sensitization treatment to enhance its ink receptivity. The material thus treated is mounted on an offset printing machine and the inked image is transferred to a print material.
In the conventional process, the silver halide emulsion layer is spectrally sensitized with a merocyanine dye or cyanine dye so as to show a sensitivity maximum at around 550 nm in the green region and subjected to exposure in a process camera to an ordinary light source such as a tungsten bulb for several seconds to several ten seconds. A printing plate made in this way exhibited only limited sharpness and resolving power, even though an aforementioned light-sensitive material inherently excellent in these properties has been used.
Besides, in reproducing a color print from a color original, the conventional process has disadvantages in that both procedures of preparing the sensitive material and making a printing plate are troublesome in addition to the insufficient resolving power.
At present, making a printing plate using laser beam and the like has been proposed as one method to solve the above-mentioned problems. U.S. Pat. No. 4,501,811 and the like disclose lithographic printing plates made using helium-neon laser, light-emitting diodes (LED), argon laser or semiconductor laser.
However, lithographic printing plates made by a high-intensity short-time exposure such as lasers in accordance with a scanning exposure technique are still insufficient in characteristics such as sensitivity, contrast, resolution, shelf stability and printing endurance and further improvements in these respects are required.
The inventors have found the surprising fact that the above-mentioned characteristics can be markedly improved in light-sensitive materials for lithographic printing plate which is to be subjected to high-intensity short-time (less than 10.sup.-4 second) such as by a laser.